1. Field of the Invention
This invention relates to methods and systems for measuring vibration in the form of time-varying displacement of an object and, in particular, to non-contact methods and systems for measuring vibration of an object.
2. Background Art
Vibration measurements are typically made by contact devices (accelerometers, strain gages, etc.) and/or non-contact devices (laser velocometers and displacometers, fiber optic probes, eddy current probes, capacitive probes, etc.).
Although these presently available systems are effective, they suffer from a number of drawbacks that hinder their effectiveness in applications where cost-effective, high resolution, non-contact measurement from a fairly large standoff distance is required.
Among the contact type devices, accelerometers give high resolution measurement over a broad range of vibration frequencies. However, since they must be mounted to bearing housings or structural mounts or casings, source vibration signals from rotating components such as shafts and rotors are attenuated before they reach the sensor. Strain gages, on the other hand, can be attached to a rotating component and therefore directly measure its vibration. However, the measurement must be transmitted off the component by cumbersome and complicated devices such as slip rings or radio telemetry.
Non-contact measurement systems offer the ability to directly sense vibration signals of rotating components; however, some characteristics of the presently available systems limit their applications. For example, fiber optic, eddy current, and capacitive systems require extremely small standoff distance (in some cases  less than 1 mm or 40 mils) from the target to achieve reasonable resolution. Laser velocity measuring devices that operate using the Doppler effect offer large standoff distance, high resolution, and high frequency response. However, at the present time these devices cost as much as $30,000 for a single point measurement system, and upwards of $100,000 for a whole field scanning system. Laser displacement devices utilizing geometric triangulation offer moderate resolution, reasonable standoff distance (150 mm or 6 in. maximum), but are also fairly costly, about $8,000 for a single point measurement system.
There has been some prior work in the use of video sensors for measuring relative velocity of targets, especially for measuring the relative velocity between two vehicles, or between a host vehicle and the ground, both of which are common in the transportation/automotive applications. However, these systems are not appropriate for laboratory or field testing of typical engineering components, as their resolution is not adequate for measuring small vibrations.
U.S. Pat. No. 4,893,922 to Eichweber, U.S. Pat. No. 5,033,096 to Morrison, et al., U.S. Pat. No. 5,173,945 to Pieters, et al., U.S. Pat. No. 5,379,106 to Baldur, U.S. Pat. No. 5,521,843 to Hashima, et al., U.S. Pat. No. 5,901,273 to Tsukamoto, et al. and U.S. Pat. No. 6,044,183 to Pryor disclose various position monitoring systems utilizing markings on a target object to calibrate the position of the object. The spaced markings have a known displacement and are monitored to determine the position of the target to which they are attached.
U.S. Pat. No. 4,044,377 to Bowerman discloses a system for the non-contact measurement of the longitudinal and lateral displacement of a target. The system is particularly adapted for permitting a robot to focus on a target for optimal operation of an automated system.
U.S. Pat. No. 1,591,612 to Dickinson and U.S. Pat. No. 4,689,993 to Slettemoen are exemplary of systems in which the vibrational movement of an object is monitored.
U.S. Pat. Nos. 4,978,222; 5,253,531 and 5,850,254 generally disclose the detection of vibration using video techniques.
U.S. Pat. Nos. 4,214,266 and 5,644,442 are of a more general interest.
An object of the present invention is to provide a method and system for measuring vibration of an object in a cost-effective, high resolution manner from a relatively large standoff distance.
Another object of the present invention is to provide a method and system for measuring vibration of an object by simultaneously imaging many points on the same object. This feature means that modal analysis (measurement of all natural frequencies and mode shapes) of a vibrating object can be achieved using a single test (called a SIMO test, for single inputxe2x80x94multiple output).
In carrying out the above objects and other objects of the present invention, a method for measuring vibration of an object, having a pair of substantially coplanar, spaced-apart marks is provided. The method includes providing calibration data based on a correlation of a change in distance between the spaced-apart marks in a detector plane substantially parallel to the spaced-apart marks to physical movement of the object including the spaced-apart marks. The method further includes generating a plurality of images including the spaced-apart marks from signals reflected from the object in the detector plane. The method further includes measuring energy in the images in the detector plane to produce a plurality of signals, and processing the plurality of signals with the calibration data to obtain a vibration measurement of the object.
The spaced-apart marks may be parallel lines which are part of the object.
The method may further include the step of marking the object with the spaced-apart marks which may be substantially coplanar parallel lines.
The signals reflected from the object may be light signals wherein the energy in the images in the detector plane is radiant energy.
Further in carrying out the above objects and other objects of the present invention, a system for measuring vibration of an object having a pair of substantially coplanar, spaced-apart marks is provided. The system includes means for generating an image including the spaced-apart marks in a detector plane substantially parallel to the spaced-apart marks. The system also includes means for storing calibration data based on a correlation of a change in distance between the spaced-apart marks in the detector plane to physical movement of the object including the spaced-apart marks. The system further includes means for generating a plurality of images including the spaced-apart marks from signals reflected from the object in the detector plane substantially parallel to the spaced-apart marks. At least one detector is provided for measuring energy in the images in the detector plane to produce a plurality of signals, and a signal processor is provided for processing the plurality of signals with the calibration data to obtain a vibration. measurement of the object.
A marker may be provided for marking the object with the spaced-apart marks such as a pair of substantially coplanar parallel lines. The substantially coplanar parallel lines may be part of the object.
The signals reflected from the object may be light signals and the at least one detector may be an array of photodetectors. The plurality of photodetectors may include a video camera.
The spaced-apart marks may be parts of a single curved line such as a sine or cosine curve or may be parts of separate lines.
The system of the present invention is a non-contact vibration measurement system with a large standoff distance and is relatively inexpensive. Such a system finds a wide range of applications in the industrial and military communities in test facilities, machinery equipment rooms, power plants, and manufacturing centers. The system is especially well suited for vibration measurement of rotating machinery components including rotors, power transmission shafts, motors, gears, flywheels, disks, and tool spindles. This market consists of the military, automotive, power, manufacturing, and aerospace industries. At the present time, comparable sensors (Laser Doppler) are very costly.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.